Can Concrete Walls Be Pinned Above Transfer Slab?

[engTC]

Hello all, I need your help regarding this.
I have seen many plans that consider axial forces (dead and live) only when evaluating transferred loads to the PT slab, ignoring the bending moments from the transferred elements. I wonder if this is the normal practice? How can you make the concrete wall (not precast) to PT transfer slab connection to be pin? I discussed this with another engineer and was told that when a building has core walls most of the lateral load will be carried by core walls so there will be not much moment on the transfer walls and as the walls reinforcement has only 300 cog to the slab so the pin connection can be achieved. I agree with the lateral load part but under gravity loads there will be much bending moment generated on the wall-slab connections and I doubt 300 cog will make it a pin connection and not adding the moments will be unsafe for the PT slab design.
So can anyone guide me how you guys evaluate the loading and design the PT slab? Do you normally add the bending moment under dead load and live load or just axial force?
Thank you.

 

[KootK]

So can anyone guide me how you guys evaluate the loading and design the PT slab? Do you normally add the bending moment under dead load and live load or just axial force

Other than for transferred shear walls, I've rarely seen an accounting of gravity induced moments in transfer slab design when the slab modelling has been performed independently of the other levels. Frankly, I suspect that this has more to do with computational efficiency than it does rigorous design logic. In a thick transfer slab, the moments at the bottom of a column should be about half what they are at the top. That said, the very nature of transfer slabs, which tends to make them thick and stiff, also tends to minimize the impact of wall and column moments coming in from the transferred structure above.

as the walls reinforcement has only 300 cog to the slab so the pin connection can be achieved.

I don't agree that a limited length cog results in any kind of real world pin. Cast in place concrete structures are inherently monolithic and continuous. In such an environment, the pinning of anything really needs to be predicated upon:

1) The relative stiffness of the various parts of the system and/or;

2) Ensuring ductile behavior such that the formation of an effective pin at some point in the load history is not a catastrophic event.

 

[engTC]

Hi KootK, Thank you very much for your help.
In my opinion, the moment at the bottom of the column/wall to transfer slab is not the same as to the fixed end. It will not be just half the top moment, in fact, I think it will be greater than the top moment. Increasing the transfer slab thickness will help reducing the moment at the bottom but as the walls above have great stiffness about major axes as well, the moment at the bottom will be quite large.
(The 3D model tells me there is 200kNm at the bottom). I have attached a pic of a 2D model (just a random example) to better illustrate my point of view.

Can you please comment on this?
Thank you very much.

 

[KootK]

My comment was meant to simply support your hypothesis that wall and column moments are not zero at the interface with the transfer slab. It was not my intention that anyone take my comments as suggesting that column bottom moments should be taken as 50% of the top moments in any particular case.

 

[slickdeals]

What is happening is that the central area with wall and beam above is acting as a stiffening vierendeel. Pinning the wall above (in the analysis) will be conservative for the design of the transfer girder. I would suggest enveloping your analysis (pinned wall for design of transfer and fixed wall for the design of wall for the post-construction loads). You will also benefit from a construction sequence analysis (although not sure if it's really necessary).

 

[KootK]

This also appears to be a case where your upper column moments would reduce with increasing transfer stiffness. Morover, it kind of is an example of the 50% algorithm. Only here, the primary column moments are induced at the bottom and carried over to the top.

 

[engTC]

Hi KootK, Thanks for your response. I agree that increasing transfer slab will reduce the moment at transfer columns/walls. As I model the structure using the real transfer slab and the moment from the transferred walls are quite a lot (I didn't pinned any walls or columns although I reduce their stiffness a bit). That's why I am concerned about the transfer slab design using the axial force only from the model.

 

[engTC]

Hi slickdeals, Thanks for your reply

Can you please explain why it is safer for transfer slab design? Take the 2D model above as an example, pinning the bottom of the transferred column above will not increase the axial force output. However, the moment will be ignored in transfer slab design, which I think it is unsafe for transfer slab design. Is there anything that I misunderstand? Thank you.

 

[slickdeals]

Take your 2D model, figure out the axial loads coming from the transfer walls. Then delete everything above including the transfer walls and apply the loads as a point load. The transfer girder moment diagram will be more like that of a beam with fixed ends (depending on what the relative stiffness of your beam/column is).

 

[engTC]

Hi slickdeals, Thanks for quick reply.
I just did as what you said, so the axial force from transferred column is 30kN. And the mid span moment in slab is around 48kNm. While the point load model shows only 39kNm only at mid span, which is less than the original model. Also, if moment is ignored here, the punching shear design will also be unsafe as far as my opinion is concerned.